Environmentally benign production of graphene oxide

ABSTRACT

A method of producing isolated graphene oxide sheets directly from a graphitic material, comprising: a) mixing multiple particles of a graphitic material, an optional oxidizing liquid, and multiple particles of a solid carrier material to form a mixture in an impacting chamber of an energy impacting apparatus; b) operating the energy impacting apparatus with a frequency and an intensity for a length of time sufficient for peeling off graphene sheets from the graphitic material and transferring the graphene sheets to surfaces of the solid carrier material particles to produce graphene-coated solid carrier particles inside the impacting chamber; and c) sequentially or concurrently oxidizing and separating the graphene sheets from the solid carrier material particle surfaces to produce isolated graphene oxide sheets. The process is fast (1-4 hours as opposed to 5-120 hours of conventional processes), has low or no water usage, environmentally benign, cost effective, and highly scalable.

FIELD OF THE INVENTION

The present invention relates to the art of graphene materials and, in particular, to a method of producing isolated graphene oxide sheets in an environmentally benign manner.

BACKGROUND

A single-layer graphene sheet is composed of carbon atoms occupying a two-dimensional hexagonal lattice. Multi-layer graphene is a platelet composed of more than one graphene plane.

Individual single-layer graphene sheets and multi-layer graphene platelets are herein collectively called nano graphene platelets (NGPs) or graphene materials. NGPs include pristine graphene (essentially 99% of carbon atoms), slightly oxidized graphene (<5% by weight of oxygen), graphene oxide (≥5% by weight of oxygen), slightly fluorinated graphene (<5% by weight of fluorine), graphene fluoride ((≥5% by weight of fluorine), other halogenated graphene, and chemically functionalized graphene.

Our research group was among the first to research graphene [B. Z. Jang and W. C. Huang, “Nano-scaled Graphene Plates,” U.S. patent application Ser. No. 10/274,473, submitted on Oct. 21, 2002; now U.S. Pat. No. 7,071,258 (Jul. 4, 2006)]. The processes for producing NGPs and NGP nanocomposites were reviewed by us [Bor Z. Jang and A Zhamu, “Processing of Nano Graphene Platelets (NGPs) and NGP Nanocomposites: A Review,” J. Materials Sci. 43 (2008) 5092-5101].

The primary approach to production of graphene oxide is based on the intercalation and oxidation of graphite, as illustrated in FIG. 1. This approach entails treating natural graphite powder with an intercalant and an oxidant (e.g., concentrated sulfuric acid and nitric acid, respectively) to obtain a graphite intercalation compound (GIC) or, actually, graphite oxide (GO). [William S. Hummers, Jr., et al., Preparation of Graphitic Oxide, Journal of the American Chemical Society, 1958, p. 1339.] Prior to intercalation or oxidation, graphite has an inter-graphene plane spacing of approximately 0.335 nm (L_(d)=1/2 d₀₀₂=0.335 nm). With an intercalation and oxidation treatment, the inter-graphene spacing is increased to a value typically greater than 0.6 nm. This is the first expansion stage experienced by the graphite material during this chemical route. The obtained GIC or GO is then subjected to further expansion (often referred to as exfoliation) using either a thermal shock exposure or a solution-based, ultrasonication-assisted graphene layer exfoliation approach.

In the thermal shock exposure approach, the GIC or GO is exposed to a high temperature (typically 800-1,050° C.) for a short period of time (typically 15 to 60 seconds) to exfoliate or expand the GIC or GO for the formation of exfoliated or further expanded graphite, which is typically in the form of a “graphite worm” composed of graphite flakes that are still interconnected with one another. This thermal shock procedure can produce some separated graphite flakes or graphene sheets, but normally the majority of graphite flakes remain interconnected. Typically, the exfoliated graphite or graphite worm is then subjected to a flake separation treatment using air milling, mechanical shearing, or ultrasonication in water. Hence, approach 1 basically entails three distinct procedures: first expansion (oxidation or intercalation), further expansion (or “exfoliation”), and separation.

In the solution-based separation approach, the expanded or exfoliated GO powder is dispersed in water or aqueous alcohol solution, which is subjected to ultrasonication. It is important to note that in these processes, ultrasonification is used after intercalation and oxidation of graphite (i.e., after first expansion) and typically after thermal shock exposure of the resulting GIC or GO (after second expansion). Alternatively, the GO powder dispersed in water is subjected to an ion exchange or lengthy purification procedure in such a manner that the repulsive forces between ions residing in the inter-planar spaces overcome the inter-graphene van der Waals forces, resulting in graphene layer separations.

There are several major problems associated with this conventional chemical production process:

-   -   (1) The process requires the use of large quantities of several         undesirable chemicals, such as sulfuric acid, nitric acid, and         potassium permanganate or sodium chlorate.     -   (2) The chemical treatment process requires a long intercalation         and oxidation time, typically 5 hours to five days.     -   (3) Strong acids consume a significant amount of graphite during         this long intercalation or oxidation process by “eating their         way into the graphite” (converting graphite into carbon dioxide,         which is lost in the process). It is not unusual to lose 20-50%         by weight of the graphite material immersed in strong acids and         oxidizers. This is also a main reason why an excessively large         amount of concentrated acid and oxidizer is required.     -   (4) The thermal exfoliation requires a high temperature         (typically 800-1,200° C.) and, hence, is a highly         energy-intensive process.     -   (5) Both heat- and solution-induced exfoliation approaches         require a very tedious washing and purification step. For         instance, typically 2.5 kg of water is used to wash and recover         1 gram of GIC, producing huge quantities of waste water that         need to be properly treated.     -   (6) Furthermore, the quantity of intercalation solution retained         on the flakes after draining may range from 20 to 150 parts of         solution by weight per 100 parts by weight of graphite flakes         (pph) and more typically about 50 to 120 pph. During the         high-temperature exfoliation, the residual intercalant species         retained by the flakes decompose to produce various species of         sulfuric and nitrous compounds (e.g., NO_(x) and SO_(x)), which         are undesirable. The effluents require expensive remediation         procedures in order not to have an adverse environmental impact.

Hence, an urgent need exists to have a graphene oxide production process that requires a reduced amount of undesirable chemical (or elimination of these chemicals all together), shortened process time, less energy consumption, lower degree of graphene oxidation, reduced or eliminated effluents of undesirable chemical species into the drainage (e.g., sulfuric acid) or into the air (e.g., SO₂ and NO₂).

Our research has yielded a process for substantially chemical-free production of isolated graphene oxide. In addition, the process is of enhanced utility in that it is cost effective, and provides graphene oxide materials with significantly reduced environmental impact.

SUMMARY OF THE INVENTION

The present invention provides a strikingly simple, fast, scalable, environmentally benign, and cost-effective process that meets the aforementioned needs. This method of producing single-layer or few layer graphene oxide directly from a graphitic or carbonaceous material (a graphene source material) comprises subjecting a mixture of graphitic material, an optional oxidizing liquid, particles of a solid carrier material, and, optionally, impacting balls to mechanical agitation via a ball mill or similar energy impacting device for a length of time sufficient for peeling off graphene layers (planes of hexagonally arranged carbon atoms) from the source graphite material, and coating these peeled-off graphene layers onto surfaces of the solid carrier material particles. With the presence of impacting balls, graphene sheets can be peeled off from the source graphite particles and tentatively deposited onto the surfaces of impacting balls. When these graphene sheet-coated impacting balls subsequently impinge upon solid carrier particles, the graphene sheets are transferred to surfaces of carrier particles. These steps are followed by sequentially or concurrently oxidizing and separating graphene sheets from solid carrier material particle surfaces to produce the isolated graphene oxide sheets.

Thus, in some embodiments, the invention provides a method of producing isolated graphene oxide sheets directly from a graphitic material, the method comprising: a) mixing multiple particles of a graphitic material, an optional oxidizing liquid, and multiple particles of a solid carrier material to form a mixture in an impacting chamber of an energy impacting apparatus; b) operating the energy impacting apparatus with a frequency and an intensity for a length of time sufficient for peeling off graphene sheets from the graphitic material and transferring the graphene sheets to surfaces of the solid carrier material particles to produce graphene-coated solid carrier particles inside the impacting chamber; and c) sequentially or concurrently oxidizing and separating the graphene sheets from the surfaces of the solid carrier material particles to produce the isolated graphene oxide sheets.

In some embodiments, a plurality of impacting balls or media are added to the impacting chamber of the energy impacting apparatus.

In some embodiments, step (a) comprises adding an oxidizing liquid in the mixture so that the oxidizing liquid acts to partially oxidize the graphene sheets during step (b). The oxidizing liquid may be selected from a liquid containing an oxidizer selected from H₂O₂, nitric acid, potassium permanganate, sodium permanganate, transition metal permanganate, sodium chlorate, potassium chlorate, or a combination thereof.

In some embodiments, step (c) comprises exposing the graphene sheets to an oxidizing medium, before, during or after the graphene sheets are separated from the solid carrier material particle surfaces, wherein the oxidizing medium is selected from an oxidizing gas or vapor, an oxidizing plasma, or an oxidizing liquid. Again, the oxidizing liquid may be selected from a liquid containing an oxidizer selected from H₂O₂, nitric acid, potassium permanganate, sodium permanganate, transition metal permanganate, sodium chlorate, potassium chlorate, or a combination thereof.

Preferably, the starting material (graphitic or carbonaceous material as a graphene source material) has never been previously intercalated or chemically oxidized. This starting material is not a graphite intercalation compound (GIC) or graphite oxide (GO). Preferably, the source graphitic material may be selected from natural graphite, synthetic graphite, highly oriented pyrolytic graphite, mesocarbon microbead, graphite fiber, graphitic nanofiber, graphite oxide, graphite fluoride, chemically modified graphite, exfoliated graphite, vein graphite, or a combination thereof.

In some embodiments, the impacting chamber of the energy impacting apparatus further contains a protective fluid; e.g. inert gas, non-reactive liquid, water, etc.

This is essentially a two-step process, significantly reducing process costs. In less than 1-4 hours of process time, graphene sheets are peeled off from graphite particles, followed by a fast, efficient removal of the carrier material. This process is stunningly short and simple.

A preferred embodiment of the present invention is a method of directly mixing a graphitic material and a carrier material into an energy impacting device, such as a ball mill, and submitting the mixture to a sufficiently long treatment time to peel off graphene layers from the source graphitic material and transfer these graphene layers immediately to the carrier material surfaces. These graphene layers are typically single-layer or few-layer graphene sheets (typically <5 layers; mostly single-layer graphene).

Following this step, the carrier layer may be removed by dissolution, burning, sublimation, melting or other method. The end result is isolated graphene sheets, which can then be oxidized by, for instance, immersing the isolated graphene sheets in an oxidizing liquid (e.g. 30% H₂O₂ in water).

In certain embodiments, the graphene sheets supported on carrier particle surfaces may be exposed to the oxidizing medium before or during the separation procedure. A very small amount of an oxidizing agent is required to accomplish the oxidation of isolated graphene sheets or graphene sheets supported on a solid carrier particle surface.

This is a surprisingly simple method. In contrast, prior researchers and manufacturers have focused on more complex, time intensive and costly methods to create graphene oxide in industrial quantities. The present invention has several unexpected attributes or advantages:

-   -   (1) Unlike the chemical intercalation and oxidation (which         requires expansion of inter-graphene spaces, further expansion         or exfoliation of graphene planes, and full separation of         exfoliated graphene sheets), the instant method directly removes         graphene sheets from a source graphitic material and transfers         these graphene sheets to surfaces of carrier material particles.         No undesirable chemicals (e.g. sulfuric acid and nitric acid)         are used in the production of isolated graphene sheets.     -   (2) Unlike conventional oxidation and intercalation methods         (e.g. well-known Hummer's method) that always produce heavily         oxidized graphene sheets (i.e. heavily damaged graphene oxide,         having an oxygen content up to 47% by weight) in a         non-controllable manner, initially pristine graphene sheets are         produced by using the invented approach. These pristine graphene         sheets can be subsequently oxidized in a controlled manner to         yield GO sheets of controllable and desirable oxygen contents         from 5% to approximately 50% by weight. The GO sheets produced         by conventional methods typically cannot recover their structure         and properties even after thermal or chemical reduction of GO         sheets. In contrast, the sheets being initially free of         oxidation damage allow the creation of graphene containing         products with higher electrical and thermal conductivity.     -   (3) The amount of sulfuric acid/oxidizer used is typically only         1-10% of what would be required for GO production using         conventional methods. Furthermore, more environmentally benign         oxidizers (e.g. H₂O₂) can be readily and easily used, instead of         undesirable chemicals such as strong sulfuric acids and nitric         acid or potassium permanganate.     -   (4) Consequently, the required amounts of washing water are         dramatically reduced. Furthermore, the amounts of NO_(x) and         SO_(x) resulting from residual sulfuric acid and nitric acid of         conventional methods are dramatically reduced or even totally         eliminated.

The carrier material can be an organic, inorganic, metal, glass, metal oxide, metal carbide, metal nitride, metal boride, or ceramic material. Carrier materials can be in the form of pellets, filament, fibers, powder, reactor spheres, or other forms. Some examples of organic carrier materials include wax, soluble polymers, nut husks, and wood chips.

The energy impacting apparatus may be selected from a ball mill, vibratory ball mill, planetary ball mill, high energy mill, basket mill, agitator ball mill, continuous ball mill, stirred ball mill, pressurized ball mill, vacuum ball mill, freezer (SPEX) mill, vibratory sieve, ultrasonic homogenizer mill, resonant acoustic mixer, or shaker table.

The presently invented process is capable of producing single-layer graphene sheets. In many examples, the graphene material produced contains at least 80% single-layer graphene sheets. The graphene produced can contain pristine graphene, oxidized graphene with less than 5% oxygen content by weight, graphene fluoride with less than 5% fluorine by weight, graphene with a carbon content no less than 95% by weight, or functionalized graphene.

Another surprising and highly advantageous feature of the presently invented process is the notion that graphene sheet production and oxidation can be accomplished concurrently in the same impacting chamber provided an oxidizing agent is enclosed in the impacting chamber. The impact-induced kinetic energy experienced by the carrier particles are of sufficient energy and intensity to chemically activate the edges and surfaces of graphene sheets coated on carrier particle surfaces; e.g. creating highly active sites or free radicals). Desired oxygen-containing groups (e.g. —COOH, —OH, etc.) can be imparted to graphene edges and/or surfaces, provided that selected chemical species (oxidizing agents) containing desired chemical function groups.) are dispersed in the impacting chamber. Chemical oxidation reactions can occur in situ as soon as the reactive sites or active radicals are formed.

The present invented method may further contain a step of functionalizing the produced graphene oxide sheets.

In some embodiments, functionalizing agents contain a chemical functional group selected from alkyl or aryl silane, alkyl or aralkyl group, hydroxyl group, carboxyl group, amine group, sulfonate group (—SO₃H), aldehydic group, quinoidal, fluorocarbon, or a combination thereof.

Alternatively, the functionalizing agent contains an azide compound selected from the group consisting of 2-azidoethanol, 3-azidopropan-1-amine, 4-(2-azidoethoxy)-4-oxobutanoic acid, 2-azidoethyl-2-bromo-2-methylpropanoate, chlorocarbonate, azidocarbonate, dichlorocarbene, carbene, aryne, nitrene, (R−)-oxycarbonyl nitrenes, where R=any one of the following groups,

and combinations thereof.

In certain embodiments, the functionalizing agent contains an oxygenated group selected from the group consisting of hydroxyl, peroxide, ether, keto, and aldehyde. In certain embodiments, the functionalizing agent contains a functional group selected from the group consisting of SO₃H, COOH, NH₂, OH, R′CHOH, CHO, CN, COCl, halide, COSH, SH, COOR′, SR′, SiR′₃, Si(—OR′—)_(y)R′₃-y, Si(—O—SiR′₂—)OR′, R″, Li, AlR′₂, Hg—X, TlZ₂ and Mg—X; wherein y is an integer equal to or less than 3, R′ is hydrogen, alkyl, aryl, cycloalkyl, or aralkyl, cycloaryl, or poly(alkylether), R″ is fluoroalkyl, fluoroaryl, fluorocycloalkyl, fluoroaralkyl or cycloaryl, X is halide, and Z is carboxylate or trifluoroacetate, and combinations thereof.

The functionalizing agent may contain a functional group is selected from the group consisting of amidoamines, polyamides, aliphatic amines, modified aliphatic amines, cycloaliphatic amines, aromatic amines, anhydrides, ketimines, diethylenetriamine (DETA), triethylene-tetramine (TETA), tetraethylene-pentamine (TEPA), polyethylene polyamine, polyamine epoxy adduct, phenolic hardener, non-brominated curing agent, non-amine curatives, and combinations thereof.

In some embodiments, the functionalizing agent contains a functional group selected from OY, NHY, O═C—OY, P═C—NR′Y, O═C—SY, O═C—Y, —CR′1-OY, N′Y or C′Y, and Y is a functional group of a protein, a peptide, an amino acid, an enzyme, an antibody, a nucleotide, an oligonucleotide, an antigen, or an enzyme substrate, enzyme inhibitor or the transition state analog of an enzyme substrate or is selected from R′—OH, R′—NR′₂, R′SH, R′CHO, R′CN, R′X, R′N⁺(R′)₃X⁻, R′SiR′₃, R′Si(—OR′—)_(y)R′_(3-y), R′Si(—O—SiR′₂—)OR′, R′—R″, R′—N—CO, (C₂H₄O—)_(w)H, (—C₃H₆O—)_(w)H, (—C₂H₂O)_(w)—R′, (C₃H₆O)_(w)—R′, R′, and w is an integer greater than one and less than 200.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 A flow chart showing the most commonly used prior art process of producing highly oxidized NGPs that entails tedious chemical oxidation/intercalation, rinsing, and high-temperature exfoliation procedures.

FIG. 2 A flow chart showing the presently invented two-step process for producing isolated graphitic materials

FIG. 3 A flow chart showing the presently invented process for producing isolated graphitic materials via a continuous ball mill.

FIG. 4(A) Transmission electron micrograph of graphene sheets produced by conventional Hummer's route (much smaller graphene sheets, but comparable thickness).

FIG. 4(B) Transmission electron micrograph of graphene sheets produced by the presently invented impact energy method.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Carbon materials can assume an essentially amorphous structure (glassy carbon), a highly organized crystal (graphite), or a whole range of intermediate structures that are characterized in that various proportions and sizes of graphite crystallites and defects are dispersed in an amorphous matrix. Typically, a graphite crystallite is composed of a number of graphene sheets or basal planes that are bonded together through van der Waals forces in the c-axis direction, the direction perpendicular to the basal plane. These graphite crystallites are typically micron- or nanometer-sized. The graphite crystallites are dispersed in or connected by crystal defects or an amorphous phase in a graphite particle, which can be a graphite flake, carbon/graphite fiber segment, carbon/graphite whisker, or carbon/graphite nanofiber.

One preferred specific embodiment of the present invention is a method of producing oxidized versions of graphene sheets or nanographene platelet (NGP) material. The NGP is essentially composed of a sheet of graphene plane (hexagonal lattice of carbon atoms) or multiple sheets of graphene plane stacked and bonded together (typically, on an average, less than five sheets per multi-layer platelet). Each graphene plane, also referred to as a graphene sheet or basal plane comprises a two-dimensional hexagonal structure of carbon atoms. Each platelet has a length and a width parallel to the graphite plane and a thickness orthogonal to the graphite plane. By definition, the thickness of an NGP is 100 nanometers (nm) or smaller, with a single-sheet NGP being as thin as 0.34 nm. However, the NGPs produced with the instant methods are mostly single-layer graphene with some few-layer graphene sheets (2-10 layers, but mostly <5 layers). The length and width of a NGP are typically between 200 nm and 20 μm, but could be longer or shorter, depending upon the sizes of source graphite material particles.

The present invention provides a strikingly simple, fast, scalable, environmentally benign, and cost-effective process that avoids essentially all of the drawbacks associated with prior art processes. As schematically illustrated in FIG. 2, one preferred embodiment of this method entails placing source graphitic material particles, an optional oxidizing agent, and carrier material particles (plus optional impacting balls, if so desired) in an impacting chamber. After loading, the resulting mixture is immediately exposed to impacting energy, which is accomplished by rotating the chamber to enable the impacting of the carrier particles (and optional impacting balls) against graphite particles. These repeated impacting events (occurring in high frequencies and high intensity) act to peel off graphene sheets from the surfaces of graphitic material particles and directly transfer these graphene sheets to the surfaces of carrier particles. This is a “direct transfer” process. If an oxidizing agent is present in the impacting chamber, oxidation of the produced graphene sheets also occurs substantially concurrently.

In certain embodiments, in the impacting chambers containing impacting balls (e.g. stainless steel or zirconia beads), graphene sheets are peeled off by the impacting balls and tentatively transferred to the surfaces of impacting balls first. When the graphene-coated impacting balls impinge upon the carrier material particles, the graphene sheets are transferred to surfaces of the carrier material particles. This is an “indirect transfer” process.

In less than 1-4 hours, most of the constituent graphene sheets of source graphite particles are peeled off, forming mostly single-layer graphene and few-layer graphene (mostly less than 5 layers). Following the direct or indirect transfer process (coating of graphene sheets on carrier material particles), the graphene sheets can be separated from the carrier material particles using a broad array of methods. For instance, the carrier material (e.g. plastic or organic material) is ignited, burning away the carrier material and leaving behind isolated nanographene platelets. The carrier material (e.g. wax, polymer, or salt) may be dissolved in a benign solvent (e.g. water, if the carrier is a water soluble material). There are many water soluble polymers (e.g. polyacrylamide and polyvinyl alcohol) and many water soluble salts (e.g. NaCl) that can be used for this purpose. If necessary, organic solvents can be used to dissolve most of the thermoplastic materials. The graphene sheets coated on metal, glass, or ceramic particles (metal oxide, metal carbide, etc.) can be easily detached from the particles by immersing the coated particles in a water bath and subjected the slurry to mild ultrasonication, for instance. Oxidation of graphene sheets can be allowed to occur before or during the detaching procedure. This graphene oxide production process is stunningly short and simple, and highly scalable.

In contrast, as shown in FIG. 1, the prior art chemical processes typically involve immersing graphite powder in a mixture of concentrated sulfuric acid, nitric acid, and an oxidizer, such as potassium permanganate or sodium perchlorate, forming a reacting mass that requires typically 5-120 hours to complete the chemical intercalation/oxidation reaction. Once the reaction is completed, the slurry is subjected to repeated steps of rinsing and washing with water and then subjected to drying treatments to remove water. The dried powder, referred to as graphite intercalation compound (GIC) or graphite oxide (GO), is then subjected to a thermal shock treatment. This can be accomplished by placing GIC in a furnace pre-set at a temperature of typically 800-1100° C. (more typically 950-1050° C.). The resulting products are typically highly oxidized graphene (i.e. graphene oxide with a high oxygen content), which must be chemically or thermal reduced to obtain reduced graphene oxide (RGO). RGO is found to contain a high defect population and, hence, is not as conducting as pristine graphene. We have observed that that the pristine graphene paper (prepared by vacuum-assisted filtration of pristine graphene sheets) exhibit electrical conductivity values in the range from 1,500-4,500 S/cm. In contrast, the RGO paper prepared by the same paper-making procedure typically exhibits electrical conductivity values in the range from 100-1,000 S/cm.

It is again critically important to recognize that the impacting process not only avoids significant chemical usage, but also produces a higher quality final product—pristine graphene as opposes to thermally reduced graphene oxide, as produced by the prior art process. Pristine graphene enables the creation of GO materials in a controlled manner.

Although the mechanisms remain incompletely understood, this revolutionary process of the present invention appears to essentially eliminate the required functions of graphene plane expansion, intercalant penetration, exfoliation, and separation of graphene sheets and replace it with an entirely mechanical exfoliation process. The whole process can take less than 1-4 hours, and can be done with no added undesirable chemicals. This is absolutely stunning, a shocking surprise to even those top scientists and engineers or those of extraordinary ability in the art.

Another surprising result of the present study is the observation that a wide variety of carbonaceous and graphitic materials can be directly processed without any particle size reduction or pre-treatment. This material may be selected from natural graphite, synthetic graphite, highly oriented pyrolytic graphite, mesocarbon microbead, graphite fiber, graphitic nanofiber, graphite oxide, graphite fluoride, chemically modified graphite, exfoliated graphite, or a combination thereof. By contrast, graphitic material for used for the prior art chemical formation and reduction of graphene oxide requires size reduction to 75 um or less average particle size. This process requires size reduction equipment (for example hammer mills or screening mills), energy input, and dust mitigation. By contrast, the energy impacting device method can accept almost any size of graphitic material. Starting material of mm or cm in size or larger has been successfully processed to create graphene. The only size limitation is the chamber capacity of the energy impacting device.

The presently invented process is capable of producing single-layer graphene oxide sheets. In many examples, the graphene material produced contains at least 80% (can be higher than 90%) single-layer graphene oxide sheets.

The presently invented process does not involve the production of GIC and, hence, does not require the exfoliation of GIC at a high exfoliation temperature (e.g. 800-1,100° C.). This is another major advantage from environmental protection perspective. The prior art processes require the preparation of dried GICs containing sulfuric acid and nitric acid intentionally implemented in the inter-graphene spaces and, hence, necessarily involve the decomposition of H₂SO₄ and HNO₃ to produce volatile gases (e.g. NO_(x) and SO_(x)) that are highly regulated environmental hazards. The presently invented process completely obviates the need to decompose H₂SO₄ and HNO₃ and, hence, is environmentally benign. No undesirable gases are released into the atmosphere during the combined graphite expansion/exfoliation/separation process of the present invention.

One preferred embodiment of the present invention is the inclusion of impacting balls or media to the impacting chamber, as illustrated in FIG. 2. The impact media may contain balls of metal, glass, ceramic, or organic materials. The size of the impacting media may be in the range from 1 mm to 20 mm, or it may be larger or smaller. The shape of the impacting media may be spherical, needle like, cylindrical, conical, pyramidal, rectilinear, or other shapes. A combination of shapes and sizes may be selected. The size distribution may be unimodal Gaussian, bimodal or tri-modal.

Another preferred embodiment of this method is removing the carrier material through burning, leaving behind isolated nanographene platelets. Unlike the high energy input required for the prior art process to reach 1050° C. to thermally reduce graphene oxide, burn out of organic materials is an exothermic process. Once initiated in a continuous oven or furnace, the energy required to maintain burn out temperature is modest and does not impact the cost effectiveness of the process.

Another preferred embodiment of this method is removing the carrier material through a mixture of burn out (oxygen atmosphere) and reduction (inert or reducing atmosphere). In this embodiment, the carrier material is ignited at room temperature or in an oven. The oven temperature is preferably 30° C. to 1000° C., more preferably 200° C.-800° C., and more preferably 350° C.-450° C. The burn out process removes 90 to 99.9% of the carrier material. In this embodiment, a second thermal process is carried out at greater than 600° C. in an inert or reducing atmosphere to remove remaining carrier material.

Another preferred embodiment of this method is removing the carrier material through melting, leaving behind isolated nanographene platelets. This process has allows for re-use of the carrier material, reducing process cost and environmental impact. Unlike the high temperatures required for the prior art process of thermal reduction of graphene oxide to produce graphene, the temperatures required for a melting process are comparatively low: for example 46° C. to 200° C. for wax, 231.9° C. for tin, 166° C. for poly lactic acid polymer. The energy input required to remove the carrier from graphene is modest and does not impact the cost effectiveness of the process.

Another preferred embodiment of this method is removing the carrier material through dissolution. For example, acrylonitrile-butadiene-styrene (ABS) carrier material can be dissolved in acetone. After washing over a filter to remove residual ABS, the filtrate can be vacuum dried, spray dried, or air dried, resulting in isolated graphene material. The solvents used in this process can be recovered via solvent recovery processes including vacuum distillation, this is a cost effective process.

Another preferred embodiment of this method is removing the carrier material through sublimation. The carrier material may be selected from ice or iodine crystals. If ice is used as the carrier material, a commercially available freeze dryer can be used to remove the carrier via sublimation. A major benefit of the sublimation approach is the avoidance of the exposure of NGP to surface tension, which can cause morphology changes or agglomeration. Another benefit of sublimation process is the ability to choose a carrier material that will leave no chemical residue on the graphene material.

One significant advantage of the present invention as compared to prior art is flexibility of selecting carrier materials. There are many opportunities to use pre-consumer or post-consumer waste material as the carrier, diverting this material from disposal by landfill or incineration. Nut shells, rice husks, shredded tires, and ground co-mingled recycled plastic are all possible cost effective carrier materials for the production of graphene. An additional advantage of this flexibility is the carrier material can be chosen to maximize burnout or to maximize solubility in low cost, environmentally friendly, and lower health hazard hazardous solvents. For example, a carrier material readily soluble in acetone may be chosen over a carrier material requiring methyl ethyl ketone for removal, and will facilitate the scale up of a cost effective manufacturing process.

An important advantage of the present invention is the opportunity to use multiple carrier removal methods. For example, melting at 200° C. may be used to remove 99% of a wax carrier material, followed by burnout at 1000° C. As another example, solvent removal may be used to remove the majority of a polymer carrier material, followed by burnout at 1200° C. The combining of carrier removal processes allows for cost effective re-use of carrier materials, while also achieving high levels of purity for the final graphene produced.

In a desired embodiment, the presently invented method is carried out in an automated and/or continuous manner. For instance, as illustrated in FIG. 3, the mixture of graphite particles and solid carrier particles (plus optional impacting balls) is delivered by a conveyer belt 12 and fed into a continuous ball mill 14. After ball milling to form graphene-coated solid carrier particles, the product mixture (possibly also containing some residual graphite particles and optional impacting balls) is discharged from the ball mill apparatus 14 into a screening device (e.g. a rotary drum 16) to separate graphene-coated solid carrier particles from residual graphite particles (if any) and impacting balls (if any). This separation operation may be assisted by a magnetic separator 18 if the impacting balls are ferromagnetic (e.g. balls of Fe, Co, Ni, or Mn-based metal). The graphene-coated carrier particles may be delivered into a combustion chamber 20, if the solid carrier can be burned off (e.g. plastic beads, rubber particles, and wax particles, etc.). Alternatively, these particles can be discharged into a dissolving chamber for dissolving the carrier particles (e.g. plastic beads). The product mass can be further screened in another (optional) screening device 22, a powder classifier or cyclone 24, and/or an electrostatic separator 26. These procedures can be fully automated.

FIG. 4(A) shows a transmission electron micrograph of graphene sheets produced by conventional Hummer's route (much smaller graphene sheets, but comparable thickness). FIG. 4(B) shows a transmission electron micrograph of graphene sheets produced by the presently invented impact energy method, which are larger in length and width.

Preferred Mode of Chemical Functionalization

Graphene sheets transferred to carrier solid particle surfaces, before or after separation, have a significant proportion of surfaces that correspond to the edge planes of graphite crystals. The carbon atoms at the edge planes are reactive and must contain some heteroatom or group to satisfy carbon valency. There are many types of functional groups (e.g. hydroxyl and carboxylic) that are naturally present at the edge or surface of graphene nanoplatelets produced through transfer to a solid carrier particle. The impact-induced kinetic energy experienced by the carrier particles are of sufficient energy and intensity to chemically activate the edges and even surfaces of graphene sheets coated on carrier particle surfaces (e.g. creating highly active sites or free radicals). These conditions enable ready oxidation of the graphene sheets (free-standing or being supported on a solid carrier surface) in the presence of an oxidizing agent in the impacting chamber. n addition, if certain chemical species containing desired chemical function groups (e.g. —NH₂, Br—, etc.) are added in the impacting chamber (preferably after oxidation of graphene occurs), these functional groups can be imparted to graphene edges and/or surfaces. In other words, production and chemical functionalization of graphene oxide sheets can be accomplished concurrently by including appropriate chemical compounds in the impacting chamber. In summary, a major advantage of the present invention over other processes is the simplicity of simultaneous production and modification of surface chemistry.

Graphene oxide sheets, after being recovered from the impacting chamber, may be functionalized through various chemical functionalization treatments. In each group of chemical species discussed below, we selected 2 or 3 chemical species for functionalization studies.

In one preferred group of chemical agents, the resulting functionalized NGP may broadly have the following formula(e): [NGP]—R_(m), wherein m is the number of different functional group types (typically between 1 and 5), R is selected from SO₃H, COOH, NH₂, OH, R′CHOH, CHO, CN, COCl, halide, COSH, SH, COOR′, SR′, SiR′₃, Si(—O—SiR′₂—)OR′, R″, Li, AlR′₂, Hg—X, TlZ₂ and Mg—X; wherein y is an integer equal to or less than 3, R′ is hydrogen, alkyl, aryl, cycloalkyl, or aralkyl, cycloaryl, or poly(alkylether), R″ is fluoroalkyl, fluoroaryl, fluorocycloalkyl, fluoroaralkyl or cycloaryl, X is halide, and Z is carboxylate or trifluoroacetate.

For NGPs to be effective reinforcement fillers in epoxy resin, the function group —NH₂ is of particular interest. For example, a commonly used curing agent for epoxy resin is diethylenetriamine (DETA), which has three —NH₂ groups. If DETA is included in the impacting chamber, one of the three —NH₂ groups may be bonded to the edge or surface of a graphene sheet and the remaining two un-reacted —NH₂ groups will be available for reacting with epoxy resin later. Such an arrangement provides a good interfacial bonding between the NGP (graphene sheets) and the matrix resin of a composite material.

Other useful chemical functional groups or reactive molecules may be selected from the group consisting of amidoamines, polyamides, aliphatic amines, modified aliphatic amines, cycloaliphatic amines, aromatic amines, anhydrides, ketimines, diethylenetriamine (DETA), triethylene-tetramine (TETA), tetraethylene-pentamine (TEPA), hexamethylenetetramine, polyethylene polyamine, polyamine epoxy adduct, phenolic hardener, non-brominated curing agent, non-amine curatives, and combinations thereof. These functional groups are multi-functional, with the capability of reacting with at least two chemical species from at least two ends. Most importantly, they are capable of bonding to the edge or surface of graphene using one of their ends and, during subsequent epoxy curing stage, are able to react with epoxide or epoxy resin at one or two other ends.

The above-described [NGP]—R_(m) may be further functionalized. This can be conducted by opening up the lid of an impacting chamber after the —R_(m) groups have been attached to graphene sheets and then adding the new functionalizing agents to the impacting chamber and resuming the impacting operation. The resulting graphene sheets or platelets include compositions of the formula: [NGP]-A_(m), where A is selected from OY, NHY, O═C—OY, P═C—NR′Y, O═C—SY, O═C—Y, —CR′1-OY, N′Y or C′Y, and Y is an appropriate functional group of a protein, a peptide, an amino acid, an enzyme, an antibody, a nucleotide, an oligonucleotide, an antigen, or an enzyme substrate, enzyme inhibitor or the transition state analog of an enzyme substrate or is selected from R′—OH, R′—NR′₂, R′SH, R′CHO, R′CN, R′X, R′N⁺(R)₃X⁻, R′SiR'₃, R′Si(—OR′—)_(y)R′_(3-y), R′Si(—O—SiR′₂—)OR′, R′—R″, R′—N—CO, (C₂H₄O—)_(w)H, (—C₃H₆O—)_(w)H, (—C₂H₄O)_(w)—R′, (C₃H₆O)_(w)—R′, R′, and w is an integer greater than one and less than 200.

The NGPs may also be functionalized to produce compositions having the formula: [NGP]—[R′-A]_(m), where m, R′ and A are as defined above. The compositions of the invention also include NGPs upon which certain cyclic compounds are adsorbed. These include compositions of matter of the formula: [NGP]—[X—R_(a)]_(m), where a is zero or a number less than 10, X is a polynuclear aromatic, polyheteronuclear aromatic or metallopolyheteronuclear aromatic moiety and R is as defined above. Preferred cyclic compounds are planar. More preferred cyclic compounds for adsorption are porphyrins and phthalocyanines. The adsorbed cyclic compounds may be functionalized. Such compositions include compounds of the formula, [NGP]—[X-A_(a)]_(m), where m, a, X and A are as defined above.

The functionalized NGPs of the instant invention can be prepared by sulfonation, electrophilic addition to deoxygenated platelet surfaces, or metallation. The graphitic platelets can be processed prior to being contacted with a functionalizing agent. Such processing may include dispersing the platelets in a solvent. In some instances the platelets may then be filtered and dried prior to contact. One particularly useful type of functional group is the carboxylic acid moieties, which naturally exist on the surfaces of NGPs if they are prepared from the acid intercalation route discussed earlier. If carboxylic acid functionalization is needed, the NGPs may be subjected to chlorate, nitric acid, or ammonium persulfate oxidation.

Carboxylic acid functionalized graphitic platelets are particularly useful because they can serve as the starting point for preparing other types of functionalized NGPs. For example, alcohols or amides can be easily linked to the acid to give stable esters or amides. If the alcohol or amine is part of a di- or poly-functional molecule, then linkage through the O- or NH-leaves the other functionalities as pendant groups. These reactions can be carried out using any of the methods developed for esterifying or aminating carboxylic acids with alcohols or amines as known in the art. Examples of these methods can be found in G. W. Anderson, et al., J. Amer. Chem. Soc. 86, 1839 (1964), which is hereby incorporated by reference in its entirety. Amino groups can be introduced directly onto graphitic platelets by treating the platelets with nitric acid and sulfuric acid to obtain nitrated platelets, then chemically reducing the nitrated form with a reducing agent, such as sodium dithionite, to obtain amino-functionalized platelets.

The following examples serve to provide the best modes of practice for the present invention and should not be construed as limiting the scope of the invention:

EXAMPLE 1 Isolated Graphene and Graphene Oxide Sheets from Flake Graphite via Polypropylene Powder-Based Carrier

In an experiment, 1 kg of polypropylene pellets, 50 grams of flake graphite, 50 mesh (average particle size 0.18mm; Asbury Carbons, Asbury N.J.) and 250 grams of magnetic stainless steel pins (Raytech Industries, Middletown Conn.) were placed in a ball mill container. The ball mill was operated at 300 rpm for 4 hours. The container lid was removed and stainless steel pins were removed via a magnet. The polymer carrier material was found to be coated with a dark carbon layer. Carrier material was placed over a 50 mesh sieve and a small amount of unprocessed flake graphite was removed.

Subsequently, two separate procedures were conducted to produce isolated graphene oxide sheets. One procedure involved placing coated carrier material in a crucible in a vented furnace at 600° C. to burn off PP for obtaining isolated graphene sheets. The graphene sheets were then immersed in a hydrogen peroxide-water solution (30% H₂O₂ in water) for a period of time ranging from 0.5 to 12 hours to produce GO having 5%-35% by wt. oxygen.

The other procedure entailed immersing graphene-coated PP particles in the same hydrogen peroxide-water solution and implementing an ultrasonication tip in the solution to concurrently oxidize and separate graphene sheets from the carrier particles for 15 minutes to 2 hours. The products were graphene oxide sheets having an oxygen content from 5 to 35% by weight.

Although polypropylene (PP) is herein used as an example, the carrier material for making isolated graphene oxide sheets is not limited to PP or any polymer (thermoplastic, thermoset, rubber, etc.). The carrier material can be a glass, ceramic, metal, or other organic material, provided the carrier material is hard enough to peel off graphene sheets from the graphitic material (if the optional impacting balls are not present).

EXAMPLE 2 Graphene Oxide from Expanded Graphite via ABS Polymer

In an experiment, 100 grams of ABS pellets, as solid carrier material particles, were placed in a 16 oz plastic container along with 5 grams of natural graphite. This container was placed in an acoustic mixing unit (Resodyn Acoustic mixer), and processed for 30 minutes. After processing, carrier material was found to be coated with a thin layer of graphene.

Graphene-coated carrier particles were then immersed in a mixture of sulfuric acid and potassium permanganate (graphene/sulfuric acid/potassium permanganate ratio of 1.0/1.0/0.5) and subjected to ultrasound energy to speed oxidation of graphene and separation of resulting GO sheets from the ABS particles. The solution was filtered using an appropriate filter and washed with distilled water. Subsequent to washing, filtrate was dried in a vacuum oven set at 60° C. for 2 hours.

It may be noted that the conventional Hummer's method typically requires the use of 24 mL (44 grams) of sulfuric acid and 5 grams of potassium permanganate to oxidize 1 gram of natural graphite. The required ratio was 1.0/44/5; i.e. significantly higher amounts of chemicals were used. Additionally, 15-30% of the natural graphite was “eaten away” during the GO production process using the Hummer's method.

EXAMPLE 3 Functionalized Graphene from Mesocarbon Micro Beads (MCMBs) via PLA

In one example, 100 grams of PLA pellets (carrier material), 2 grams of MCMBs (China Steel Chemical Co., Taiwan), and 2 grams of carboxylic acid (an oxidizing agent) were placed in a vibratory ball mill, which also contains particles of magnetic stainless steel impactor and processed for 1 hour to obtain graphene oxide-coated PLA particles. Subsequently, DETA was added and the material mixture was processed for an additional 1 hour. The vibratory mill was then opened and the carrier material was found to be coated with a brown-color coating of graphene oxide. The magnetic steel particles were removed with a magnet. The carrier material was rinsed with isopropyl alcohol and placed on a vacuum filter. The vacuum filter was heated to 160° C. and vacuum was applied, resulting in removal of the PLA.

In separate experiments, the following functional group-containing species were introduced to the graphene oxide sheets produced: an amino acid, sulfonate group (—SO₃H), 2-azidoethanol, polyamide (caprolactam), and aldehydic group. In general, these functional groups were found to impart significantly improved interfacial bonding between resulting graphene oxide sheets and epoxy, polyester, polyimide, and vinyl ester matrix materials to make stronger polymer matrix composites. The interfacial bonding strength was semi-quantitatively determined by using a combination of short beam shear test and fracture surface examination via scanning electronic microscopy (SEM). Non-functionalized graphene sheets tend to protrude out of the fractured surface without any residual matrix resin being attached to graphene sheet surfaces. In contrast, the fractured surface of composite samples containing functionalized graphene oxide sheets do not exhibit any bare graphene sheets; any what appears to be graphene sheets were completely embedded in a resin matrix.

EXAMPLE 4 Graphene Oxide from HOPG via Glass Beads and SPEX Mill

In an experiment, 5 grams of glass beads were placed in a SPEX mill sample holder (SPEX Sample Prep, Metuchen, N.J.) along with 0.25 grams of HOPG derived from graphitized polyimide. This process was carried out in a 1% “dry room” to reduce the condensation of water onto the completed product. The SPEX mill was operated for 10 minutes. After operation, the contents of the sample holder were transferred to a water bath subjected to ultrasonication, which helps to separate graphene sheets from glass bead surfaces. The remaining material in the weight dish was a mixture of single-layer graphene (86%) and few layer graphene. The graphene sheets were then oxidized in a hydrogen peroxide-water solution (30% H₂O₂ in water) for 0.5-2.0 hours.

EXAMPLE 5 Production of Few Layer Graphene Oxide via Wax-Based Carrier

In one example, 100 grams of hard machining wax in pellet form (F-14 green, Machinablewax.com, Traverse City Mich., Hardness 55, Shore “D”) was mixed with 10 grams of vein graphite (40 mesh size, Asbury Carbons, Asbury N.J.) and loaded into a vibratory mill. The material was processed for 4 hours, and the vibratory mill was opened. The wax pellets were found to be carbon coated. These pellets were removed from the mill, melted, and re-pelletized, resulting in 103 grams of graphene-loaded wax pellets. The wax pellets were placed again in the vibratory mill with an additional 10 grams of vein graphite, and processed for 4 hours. The resultant material was pelletized and processed with an additional 10 grams of vein graphite, creating a wax/graphene composite with a graphene filler level of about 8.9%. The wax carrier material was then dissolved in hexane and transferred into acetone via repeated washing, then separated from acetone via filtration, producing isolated, pristine NGP. The graphene sheets were dried in a vacuum oven at 60° C. for 24 hours, and then surface area was measured via nitrogen adsorption BET. The graphene sheets were then oxidized in sodium chlorate-water solution for 0.5 hours.

EXAMPLE 6 Low Temperature Metal Particles as the Carrier Material

In one example, 100 grams of tin (45 micron, 99.9% purity, Goodfellow Inc.; Coraopolis, Pa.) was mixed with 10 grams of vein graphite (40 mesh size, Asbury Carbons, Asbury N.J.) and loaded into a vibratory mill. The material was processed for 2 hours, and the vibratory mill was opened. The tin powder was found to be carbon coated. These pellets were removed from the mill and poured into a hydrogen peroxide-water solution (30% H₂O₂ in water), subjected to ultrasonication. The resulting graphene oxide sheets were filtered using a Teflon membrane. The specific surface area of the resulting graphene oxide was measured via nitrogen adsorption BET. A similar procedure was conducted using zinc particles as the solid carrier material.

EXAMPLE 7 Isolated Graphene and Graphene Oxide from Natural Graphite Particles via Polyethylene (PE) Beads and Ceramic Impacting Balls

In an experiment, 0.5 kg of PE beads (as a solid carrier material), 50 grams of natural graphite (source of graphene sheets) and 250 grams of zirconia powder (impacting balls) were placed in containers of a planetary ball mill. The ball mill was operated at 300 rpm for 4 hours. The container lid was removed and zirconia beads (different sizes and weights than graphene-coated PE beads) were removed through a vibratory screen. The polymer carrier material was found to be coated with a dark carbon layer. Carrier material was placed over a 50 mesh sieve and a small amount of unprocessed flake graphite was removed. Coated carrier material was then placed in a crucible in a vented furnace at 600° C. After cooling down, the furnace was opened to reveal a crucible full of isolated graphene sheet powder (>95% single-layer graphene). The resulting graphene sheets were then exposed to Oleum (—SO₃) vapor for oxidation for 1 hour to produce graphene oxide sheets. 

1. A method of producing isolated graphene oxide sheets directly from a graphitic material, said method comprising: a) mixing multiple particles of a graphitic material, an optional oxidizing liquid, and multiple particles of a solid carrier material to form a mixture in an impacting chamber of an energy impacting apparatus; b) operating said energy impacting apparatus with a frequency and an intensity for a length of time sufficient for peeling off graphene sheets from said graphitic material and transferring said graphene sheets to surfaces of said solid carrier material particles to produce graphene-coated solid carrier particles inside said impacting chamber; and c) sequentially or concurrently oxidizing and separating said graphene sheets from said surfaces of said solid carrier material particles to produce said isolated graphene oxide sheets.
 2. The method of claim 1, wherein a plurality of impacting balls or media are added to the impacting chamber of said energy impacting apparatus.
 3. The method of claim 1, wherein said step (a) comprises adding said oxidizing liquid in said mixture so that said oxidizing liquid acts to partially oxidize said graphene sheets during step (b).
 4. The method of claim 1, wherein said step (c) comprises exposing said graphene sheets to an oxidizing medium, before, during or after the graphene sheets are separated from said solid carrier material particle surfaces, wherein said oxidizing medium is selected from an oxidizing gas or vapor, an oxidizing plasma, or an oxidizing liquid.
 5. The method of claim 4, wherein said oxidizing liquid is selected from a liquid comprising an oxidizer selected from H₂O₂, nitric acid, potassium permanganate, sodium permanganate, transition metal permanganate, sodium chlorate, potassium chlorate, or a combination thereof.
 6. The method of claim 1, wherein said solid carrier material is selected from solid particles of an organic, polymeric, metal, glass, ceramic, or inorganic material.
 7. The method of claim 1, wherein said solid carrier material is selected from the group consisting of plastic beads, plastic pellets, wax pellets, polymer powder or polymer reactor spheres, glass beads or fibers, metal particles or wires, metal oxide particles, ceramic particles, and combinations thereof.
 8. The method of claim 1, wherein said solid carrier material includes micron- or nanometer-scaled particles that can be dissolved in a solvent, melted above a melting temperature, etched away using an etching agent, vaporized or sublimed away, or burned off, and said step (c) includes a procedure of dissolving, melting, etching, vaporizing, subliming, or burning off said solid carrier material for separating said graphene sheets.
 9. The method of claim 1, wherein said step (c) includes a procedure of dissolving, melting, etching, vaporizing, subliming, burning off, or ultrasonicating said solid carrier material for separating said graphene sheets.
 10. The method of claim 1, wherein said step (c) comprises oxidizing said graphene sheets on said solid carrier material particle surfaces in an oxidizing liquid medium while being submitted to ultrasonication.
 11. The method of claim 1 wherein said graphitic material is selected from the group consisting of natural graphite, synthetic graphite, highly oriented pyrolytic graphite, graphite fiber, graphitic nanofiber, graphite fluoride, oxidized graphite, chemically modified graphite, exfoliated graphite, recompressed exfoliated graphite, expanded graphite, mesocarbon microbead, and combinations thereof.
 12. The method of claim 1, wherein the energy impacting apparatus is a vibratory ball mill, planetary ball mill, high energy mill, basket mill, agitator ball mill, continuous ball mill, stirred ball mill, pressurized ball mill, freezer mill, vibratory sieve, ultrasonic homogenizer mill, or resonant acoustic mixer.
 13. The method of claim 1, wherein said graphitic material contains a non-intercalated and non-oxidized graphitic material that has never been previously exposed to a chemical or oxidation treatment prior to said mixing step.
 14. The method of claim 1 wherein said graphene oxide sheets contain single-layer graphene oxide sheets.
 15. The method of claim 1 wherein said graphene oxide sheets contain at least 80% single-layer graphene oxide or at least 80% few-layer graphene oxide having no greater than 10 graphene planes.
 16. The method of claim 1, further comprising a step of exposing said graphene oxide to a functionalizing agent to produce functionalized graphene oxide.
 17. The method of claim 16 wherein said functionalizing agent contains a chemical functional group selected from the group consisting of alkyl or aryl silane, alkyl or aralkyl group, hydroxyl group, carboxyl group, amine group, sulfonate group (—SO₃H), aldehydic group, quinoidal, fluorocarbon, and combinations thereof.
 18. The method of claim 16, wherein said functionalizing agent contains an azide compound selected from the group consisting of 2-azidoethanol, 3-azidopropan-1-amine, 4-(2-azidoethoxy)-4-oxobutanoic acid, 2-azidoethyl-2-bromo-2-methylpropanoate, chlorocarbonate, azidocarbonate, dichlorocarbene, carbene, aryne, nitrene, (R−)-oxycarbonyl nitrenes, where R=any one of the following groups,

and combinations thereof.
 19. The method of claim 16, wherein said functionalizing agent contains an oxygenated group selected from the group consisting of hydroxyl, peroxide, ether, keto, and aldehyde.
 20. The method of claim 16, wherein said functionalizing agent contains a functional group selected from the group consisting of SO₃H, COOH, NH₂, OH, R′CHOH, CHO, CN, COCl, halide, COSH, SH, COOR′, SR′, SiR′₃, Si(—OR′—)_(y)R′₃-y, Si(—O—SiR′₂—)OR′, R″, Li, AlR′₂, Hg—X, TlZ₂ and Mg—X; wherein y is an integer equal to or less than 3, R′ is hydrogen, alkyl, aryl, cycloalkyl, or aralkyl, cycloaryl, or poly(alkylether), R″ is fluoroalkyl, fluoroaryl, fluorocycloalkyl, fluoroaralkyl or cycloaryl, X is halide, and Z is carboxylate or trifluoroacetate, and combinations thereof.
 21. The method of claim 16, wherein said functionalizing agent contains a functional group selected from the group consisting of amidoamines, polyamides, aliphatic amines, modified aliphatic amines, cycloaliphatic amines, aromatic amines, anhydrides, ketimines, diethylenetriamine (DETA), triethylene-tetramine (TETA), tetraethylene-pentamine (TEPA), polyethylene polyamine, polyamine epoxy adduct, phenolic hardener, non-brominated curing agent, non-amine curatives, and combinations thereof.
 22. The method of claim 16, wherein said functionalizing agent contains a functional group selected from OY, NHY, O═C—OY, P═C—NR′Y, O═C—SY, O═C—Y, —CR′1-OY, N′Y or C′Y, and Y is a functional group of a protein, a peptide, an amino acid, an enzyme, an antibody, a nucleotide, an oligonucleotide, an antigen, or an enzyme substrate, enzyme inhibitor or the transition state analog of an enzyme substrate or is selected from R′—OH, R′—NR′₂, R′SH, R′CHO, R′CN, R′X, R′N⁺(R)₃X⁻, R′SiR′₃, R′Si(—OR′—)_(y)R′_(3-y), R′Si(—O—SiR′₂—)OR′, R′—R″, R′—N—CO, (C₂H₄O—)_(w)H, (—C₃H₆O—)_(w)H, (—C₂H₄O—)_(w)—R′, (C₃H₆O)_(w)—R′, R′, and w is an integer greater than one and less than
 200. 23. The method of claim 1, wherein said procedure of operating said energy impacting apparatus is conducted in a continuous manner using a continuous energy impacting device. 